Ernie was uncomfortable that Susan was uncomfortable. There wasn't much he could do really, everything had been in motion for a full week now at least. It wasn't like it was someting he could go back on, especially not when it was a Slytherin.
"I know it doesn't make it much better," he conceded, knowing he had thought out this arguement with himself too, "but my dad was in his dad's year, and he's never had reason to complain about him, and he's not some crazy Death Eater. Still a Slytherin, and I know I need to watch out, but my dad deals with their lot frequently enough. And I'm not putting any sensitive information in their hands." He felt like he was trying to convince himself that he was still himself just as much as he was trying to reassure Susan.
"So yes, that sort of would explain it." He sighed. "As for suspects, Miss Weasley and Miss Capper has been avoiding me lately as well as being loudly against the Squid's departure and pulling the previous prank on the Slytherins. And I imagine the Weasley twins would be the ideal source for itching powder, although it is not to be found in their catalogue." Ernie very delibrately did not answer Susan's question about his methods, mostly because somewhere along the line he had decided that he was not cut out for detective work.